Heat-resistant resins typified by polyimide resins have been widely used as surface protective layers and interlayer insulation films in semiconductor elements in the electronics field because of their excellent heat resistance and mechanical properties. Patent Literatures 1 and 2 disclose in detail processes for producing primary passivation layers and secondary passivation layers formed on surfaces in power semiconductor elements.
In recent years, with the compaction, thinning and high integration of semiconductor devices, insulation and protection of small areas became necessary and formation of protective layers and the like having a precise pattern is demanded. That is, formation of layers became necessary which give protection from α-ray and external stresses such as the pressure applied during resin molding, which became great enemy of semiconductor devices because of the increased precision of the semiconductor devices. As for flash memories, necessity of protective layers is increasing because of the increasing capacity thereof. On the other hand, as for the protective layers themselves, with the thinning of semiconductor wafers in recent years, in order to prevent the warping during the formation of protective films, demands for using a low-stress protective material and/or process of forming a protective film only on the required area in the semiconductor are increasing.
In the conventional production techniques, a method of forming a protective layer in a semiconductor device in which a polyamic acid or a polyimide resin varnish for protective layers is coated by spin coating method to form a thin film has been used in practice. However, this method has a problem in that it is difficult to control the film thickness to a thickness of not less than 10 μm, although a thin film with a thickness of several micrometers can be easily formed. Further, with this method, only a part of the fed material is used for forming the film, and most of the fed material is discarded, so that the yield of the method is very low. Still further, this method has a problem in that it cannot form a thin layer only on the necessary area. Therefore, an additional patterning step for forming a desired pattern, such as photolithography is necessary for forming the thin film only on the necessary area, which is complicated.
Still further, in most cases, the polyimide resin used for forming the polyimide layer is in the form of a polyamic acid. To convert the polyamic acid to polyimide, a step of heating the coated thin film to ring-close (imidize) the polyamic acid is necessary. As a result, the method has a problem in the processability, such as the shrinkage of the resin during the imidization reaction is large, so that it is difficult to form a resin protective layer with a precise pattern particularly on a semiconductor wafer or the like.
A method of forming a resin pattern by exposure with light using a photosensitive polyimide resin has also been proposed. However, this method has a problem in that the photosensitizing material is limited and expensive, and the method may not be used in wet system. Further, in many cases, in power semiconductor elements, the outgas components attach to the metal layers which are to be connected mainly by solder, so that the wettability of the solder is decreased. As a result, there is a concern that the reliability of the product may be reduced, so that the photosensitizing material cannot be used.
Recently, as the methods of forming images of polyimide resin films used as surface protective films, interlayer insulation films, stress buffers and the like, screen printing method and dispense method are attracting attention. These methods do not require complicated steps such as exposure, development, etching and the like, and films can be formed only on the necessary portions on wafers. As a result, a large cost saving can be attained.
Patent Literature 3 discloses, as a method of forming a protective film on the surface of a semiconductor wafer, a method of coating the surface of the wafer with a paste for printing by screen printing method. The paste is composed of a polyimide which is a base resin, an inorganic filler such as silica, and a solvent. The inorganic filler is added for giving thixotropic property, so as to prevent the sagging and bleeding during printing. However, since a large amount of the inorganic filler is added, there is a tendency that problems in that the film strength is decreased, adhesion with the substrate is decreased and the like arise. Further, since N-methyl-2-pyrrolidone (NMP) is used as the solvent, paste and the screen are largely influenced. That is, due to moisture absorption by NMP, the viscosity of an NMP-containing paste changes and in a severe case, even the resin component is precipitated. Once the resin component precipitates, mesh of the screen is clogged, and when the viscosity change occurs, the printing conditions change with time, so that stable printing cannot be attained. As for the screen, since the resistance of the emulsion to NMP is low, dimensional change of the pattern and skipping and chipping of small pattern occur, which give bad influence on the products. These problems are more and more severe as the pattern is finer and finer. The above-described problems of NMP cannot be solved by decreasing the NMP content, and in many cases, even when the NMP content is considerably small, NMP gives influence. As a result, there is a tendency that the NMP-containing paste is a special paste which can be handled only by a skilled person who thoroughly knows the characteristics of the paste.
To solve the problems caused by the inorganic fillers, Patent Literatures 4 to 6 propose heat-resistant resin pastes by which polyimide patterns having excellent properties can be formed by composing the resin composition with a special organic filler which melts during heating and drying, is dissolved in the base resin and forms the film together with the base resin (i.e., soluble filler), a base resin and a solvent. However, since the viscosity at 25° C. is 100 to 10,000 Pa·s, which is relatively high, there is a problem in that the screen mesh is not easily detached from the wafer, so that continuous printing is difficult.